Plugin Screenshots Downloading?

It’s not you, it’s SVNSVN Short for "SubVersioN", it's the code management system used to maintain the plugins hosted on WordPress.org. It's similar to git.. And you.

The tl;dr is that the MIME Types on your images aren’t correct. Per Otto:

On Windows, using TortoiseSVN, you can right click the screenshot file, and select the TortoiseSVN->Properties menu. There you will find the svn:mime-type property, probably incorrectly set to “application/octet-stream”. Change that to the proper mime type of “image/png” or “image/jpg” accordingly. Afterwards, commit the change.

If you prefer command line SVN:

svn propset svn:mime-type image/png screenshot-1.png
svn commit

Or similar. If somebody uses a different SVN client, look for “properties” and then the svn:mime-type property.

Once you do that, upload the changes and go make coffee. The information will sync out, but it may take a little while.